1. Field of the Invention
The present invention relates to a Class-D amplifier circuit that drives a speaker or headphones.
2. Description of the Related Art
A Class-D amplifier circuit is employed in order to amplify a weak audio signal so as to drive an electroacoustic conversion element such as a speaker, headphones, or the like. FIG. 1 is a circuit diagram showing an output stage of a Class-D amplifier circuit. The Class-D amplifier circuit 100R includes a half-bridge output stage 102, driving circuits 104H and 104L, and a pulse width modulator 106.
The output stage 102 includes a high-side transistor MH arranged between a power supply pin VCC and an output pin OUT and a low-side transistor ML arranged between the output pin OUT and a ground pin GND. The OUT pin is coupled to an electroacoustic conversion element 202 via an LC filter 204 and an output coupling capacitor 205.
The pulse width modulator 106 receives an analog or otherwise digital audio signal, and generates a PWM signal having a duty ratio (pulse width) that changes according to the audio signal. The driving circuits 104H and 104L drive the high-side transistor MH and the low-side transistor ML, respectively, according to the PWM signal generated by the pulse width modulator 106.
If a large current flows through the output stage 102, such an arrangement has the potential to involve degraded reliability of the transistors MH and ML which are circuit elements of the output stage 102. In order to solve such a problem, the output stage 102 of the Class-D amplifier circuit 100R is provided with overcurrent protection circuits 120H and 120L.
The overcurrent protection circuit 120H compares a current IMH that flows through the high-side transistor MH with an overcurrent detection threshold value IOCP. When the current IMH that flows through the high-side transistor MH exceeds the threshold value IOCP, judgment is made that an overcurrent state has occurred. In this case, the high-side transistor MH is forcibly turned off. Similarly, when a current IML that flows through the low-side transistor ML exceeds the threshold value IOCP, the overcurrent protection circuit 120L judges that an overcurrent state has occurred. In such an overcurrent state, the low-side transistor ML is forcibly turned off
The overcurrent protection circuit 120H (120L) is vulnerable to the effects of switching noise that occurs in the output stage 102. In order to solve such a problem, a predetermined judgment time τ1 is defined. When a state in which IMH>IOCP continues for only a period that is shorter than the judgment time τ1, such a state is masked. Only when a state in which IMH>IOCP continues for a period that is longer than the judgment time τ1, judgment is made that overcurrent protection is to be performed. This suppresses the effects of noise.
FIGS. 2A and 2B are waveform diagrams each showing the overcurrent protection operation. First, description will be made with reference to FIG. 2A regarding an ordinary overcurrent protection operation. For ease of understanding, description will be made directing attention to the inductor L of the LC filter 204 that functions as a load of the Class-D amplifier circuit 100R. Typically, the following expression (1) holds true between the current IOUT that flows through an inductor and a voltage v across the inductor.IOUT=1/L×∫v dt   (1)
Accordingly, assuming that the voltage v is constant, the output current IOUT increases with a constant slope according to the passage of time. With the half-bridge Class-D amplifier shown in FIG. 1, when the high-side transistor MH is turned on, the following relation holds true. That is to say, v is approximately equal to VCC (which is approximation assuming that the voltage across the electroacoustic conversion element 202 is zero). Thus, the following expression (2) holds true.IOUT=VCC/L×t   (2)
In FIG. 2A, in a case in which the output current IOUT becomes larger than the threshold value IOCP at the time point t0, after the judgment time τ1 has elapsed, i.e., at the time point t1, the overcurrent protection is enabled (OCP is set to the high level). In the overcurrent protection state, the high-side transistor MH is forcibly turned off, and accordingly, the output current IOUT is blocked. In the judgment time τ1, the output current IOUT rises as represented by ΔI=VCC/L×τ1.
As a result of investigating the Class-D amplifier circuit 100R shown in FIG. 1, the present inventor has come to recognize the following problem.
In a case in which a DC (bias) current is applied to an inductor, magnetic saturation occurs, which leads to a reduction in the inductance value. This is known as the DC superposition characteristics of an inductor. FIG. 3 is a diagram for describing the DC superposition characteristics of the inductor. Specifically, in a range in which the DC current is smaller than an allowable current IDC_MAX, the inductance value exhibits a substantially constant value. When the DC current exceeds the allowable current IDC_MAX, the inductance value suddenly falls.
FIG. 2B shows the operation when a DC current that flows through the inductor L of the LC filter 204 becomes larger than the allowable current IDC_MAX. When magnetic saturation occurs, the inductance value L falls (the inductance value in this state will be represented by L′). In this case, the output current IOUT represented by Expression (2) has a steep slope.
In a case in which the output current IOUT exceeds the threshold value IOCP at the time point t0, after the judgment time τ1 elapses in this state, i.e., at the time point t1, the overcurrent protection is enabled. However, in the judgment time τ1, the output current IOUT rises as represented by ΔI=VCC/L′×τ1. Accordingly, such an arrangement has the potential to allow deviation of the output current IOUT at the time point t1 from an assurance range in which the reliability of the circuit elements can be ensured, to a damage range represented by the hatched area.
It should be noted that similar problems can occur in Class-D amplifiers employing the BTL (Bridged Transformerless) method.